Fan tray perforation pattern

ABSTRACT

An apparatus is provided in one example embodiment and includes a plate having a plurality of perforations configured in a pattern. The pattern includes the plurality of perforations arranged in concentric circles centered at a point. Each of the perforations is a closed shape comprising four edges, with rounded corners between adjacent edges, with two opposite edges of each of the perforations including non-parallel straight lines and two other opposite edges comprise concentric, offset curved lines. The non-parallel straight lines may form an angle with a vertex at the point, and the concentric curved lines may be centered at the point. The perforations in each concentric circle may be angularly spaced around the point. The apparatus may further include a substantially circular fan.

TECHNICAL FIELD

This disclosure relates in general to the field of computer andnetworking systems and, more particularly, to a fan tray perforationpattern.

BACKGROUND

Over the past several years, information technology (IT) has seen atremendous increase in performance of electronic equipment, coupled witha geometric decrease in floor space to house the equipment. Further,increased performance requirements have led to increased energy use aswell, resulting in increased heat dissipation within the crowded floorspace. For example, the rate of increase of heat density forcommunications equipment was 13% annually from 1992 through 1998, atwhich time it increased to 28%, and is projected to continue toincrease. As a result, data centers are demanding better thermallymanaged products that have good computing performance coupled with goodthermal performance. Thus, there is a need to design electronicequipment with better thermal characteristics.

BRIEF DESCRIPTION OF THE DRAWINGS

To provide a more complete understanding of the present disclosure andfeatures and advantages thereof, reference is made to the followingdescription, taken in conjunction with the accompanying figures, whereinlike reference numerals represent like parts, in which:

FIG. 1 is a simplified block diagram illustrating an exploded view of afan tray assembly according to an example embodiment;

FIGS. 2A and 2B are simplified diagrams illustrating example details ofthe fan tray assembly in accordance with one embodiment;

FIG. 3 is a simplified diagram illustrating other example detailsassociated with an example embodiment of the fan tray assembly;

FIGS. 4A to 4G are simplified diagrams illustrating other exampledetails of the fan tray assembly in accordance with an embodiment;

FIG. 5 is a simplified flow diagram illustrating example operations thatmay be associated with an embodiment of the fan tray assembly;

FIG. 6 is a simplified flow diagram illustrating yet other exampleoperations that may be associated with an embodiment of the fan trayassembly; and

FIG. 7 is a simplified flow diagram illustrating yet other exampleoperations that may be associated with an embodiment of the fan trayassembly.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS Overview

An apparatus is provided in one example embodiment and includes a platehaving a plurality of perforations configured in a pattern. The patternincludes the plurality of perforations arranged in concentric circlescentered at a point. Each of the perforations is a closed shapecomprising four edges, with rounded corners between adjacent edges, withtwo opposite edges of each of the perforations including non-parallelstraight lines and two other opposite edges include concentric, offsetcurved lines. The non-parallel straight lines may form an angle with avertex at the point, and the concentric curved lines may be centered atthe point. The perforations in each concentric circle may be(substantially) angularly spaced around the point.

In a specific embodiment, the apparatus may further include asubstantially circular fan having an inner radius corresponding to aninner perimeter of the pattern and an outer radius corresponding to anouter perimeter of the pattern. A smallest one of the concentric circlesmay have a radius that is approximately equal to the inner radius and alargest one of the concentric circles may have a radius that isapproximately equal to the outer radius, such that the pattern overlapsan area of the fan between the inner radius and the outer radius. Whenthe fan rotates, air may be pushed out through the plurality ofperforations. In specific embodiments, the plate may comprise a metallicmaterial with an electromagnetic interference (EMI) shielding. Theapparatus may be attached to electronic equipment, such as a switch in anetwork.

EXAMPLE EMBODIMENTS

Turning to FIG. 1, FIG. 1 is a simplified block diagram illustrating anexploded view of a fan tray assembly 10 in accordance with one exampleembodiment. Fan tray assembly 10 includes a substantially circular fan12, and a plate 14, which includes a plurality of perforations 16. Asused herein, the term “perforation” includes holes, punctures, and otherapertures. The term “plate” can include a relatively thin, rigid body ofsubstantially uniform thickness. Plate 14 may be smooth or flat, and ofany desired shape. Plate 14 may include attachments to fasten to otherparts of fan tray assembly 10, including fins, brackets, screws, andother fastening mechanisms. In some embodiments, plate 14 may form aportion of an otherwise non-uniform three-dimensional component (e.g.,stamped sheet metal enclosure). In some embodiments, plate 14 may bewelded or otherwise irremovably attached to fan 12. In otherembodiments, plate 14 may be detachably attached to fan 12.

Fan assembly 10 may include other parts, such as housing for fan 12, fanguards, and electrical wiring. Fan tray assembly 10 may be included as apart of electronic equipment. For example, fan tray assembly 10 may bepart of a chassis of a switch in a network. In another example, fan trayassembly 10 may be part of a chassis of a computer. In yet otherexample, fan tray assembly 10 may be part of a power supply unit in anetwork, and so on. In general, fan tray assembly 10 may be used to coolthe computer or networking equipment of which it is a part. Hot air mayenter a front 18 of fan 12 and be forced out, by the fan's action,through plurality of perforations 16.

For purposes of illustrating the techniques of fan tray assembly 10, itis important to understand the elements in a given system such as thesystem shown in FIG. 1. The following foundational information may beviewed as a basis from which the present disclosure may be properlyexplained. Such information is offered earnestly for purposes ofexplanation only and, accordingly, should not be construed in any way tolimit the broad scope of the present disclosure and its potentialapplications.

Modular ventilation fan assemblies, called “fan trays” are used to mountcooling fans to electronic enclosures, such as switch chassis, computerchassis, etc. The fan trays include fans mounted in conductiveenclosures to contain electromagnetic interference (EMI) generated bythe electronic equipment. The fans are generally used for thermalmanagement, to cool the electronic equipment. One or more fans may beincluded in each fan tray, depending on the cooling needs of theelectronic equipment.

Fan trays typically provide metal plates on opposite sides of the fan toelectromagnetically isolate the fan from the outside environment and theelectronic equipment. The metal plates are perforated to allow forairflow. The higher the number of perforations in the metal plates, thelower the electromagnetic interference (EMI) shielding properties of themetal plates, which is not desired. However, lowering the number ofperforations in the metal plates, lowers the thermal cooling propertiesof the fan tray, which is also not desired. Thus, there is a tradeoffbetween EMI shielding properties and thermal cooling properties whenconfiguring the perforations on the metal plates.

Additionally, manufacturing constraints also affect perforationconfiguration. One method of making the perforations on the metal plateinvolve punching out the holes using a punch fabricated according to theperforation pattern. Another method involves punching out holes in themetal plates individually, one at a time, to achieve the desiredpattern. Yet another method involves laser drilling the holes in thedesired pattern. Various other methods may also be used as needed. Eachof the manufacturing methods involves associated capital and operatingcosts.

Typical patterns of perforations include simple round or hexagonalperforations patterned in Cartesian co-ordinate system along straight Xand Y directions (corresponding to the length and width of the metalplates). Such a pattern cannot match a fan's round shape, and thereforecannot provide sufficient airflow. If the patterns are cut out in theshape of the fan (with perforations along the perimeter of the fan'sboundary being partially cut), the pattern is not aesthetically pleasingand cannot be manufactured in a cost-effective manner (e.g., faster toolwear out, higher wastage of materials, etc.). If the pattern is made inthe cylindrical co-ordinate system, along the radial direction, forexample, in simple geometric shapes (e.g., circle, square, hexagon,etc.), the pattern may match the fan's round shape, but may not providesufficient EMI shielding or airflow. Moreover, the pattern may not fullyutilize substantially all the available cutout space. Thus, the typicalfan tray perforation area has uneven and non-uniform pattern that isboth aesthetically unappealing and compromises airflow/EMI performance.

Fan tray assembly 10 is configured to address these issues (and others)in offering a fan tray perforation pattern 20 with enhanced air flow andbetter EMI shielding performance (among other advantages). Embodimentsof fan tray assembly 10 can include pattern 20 comprising plurality ofperforations 16. Pattern 20 can provide enhanced airflow through thefan, while minimizing EMI, conserving material, and providing anaesthetically pleasing design. Each of the perforations in pattern 20may include a unique shape that may be a combination of straight linesand large curvature lines, patterned in a radial direction. Pattern 20can provide electricity saving due to the fans running at lower speed,which can contribute to a greener environment with large savings inenergy usage.

Each individual perforation in pattern 20 may be created from two largecurvature lines and two straight lines. Then the individual perforationmay be repeated in a radial direction by certain angle increments. Afterone layer of perforations is completed, the perforations may be repeatedinto a next layer with an incrementally larger radius. The number ofperforations for the layers may be different from each other for optimalfill pattern and to maintain the uniform wall thickness between any twoperforations (e.g., to improve manufacturability and airflow). Thelayers are repeated until the fan boundary. Resulting pattern 20 may beboth beautiful and more functional than traditional fan tray patterns.

Embodiments of fan tray assembly 10 may have several advantages. Pattern20 may match the fan's round shape, and may be aesthetically farsuperior to the traditional fan tray perforation patterns. The simplebut elegant design of pattern 20 may be pleasing to look at. Pattern 20can also provide a minimum 60% air-opening ratio whereas the traditionalX-Y pattern offers around 55% at best. Moreover, pattern 20 may alsooutperform the traditional fan tray perforation by providing better EMI.For example, pattern 20 can provide at least 2 dB improvement in EMIcompared to the traditional pattern. In some embodiments, pattern 20 mayoffer an aesthetic design that provides sufficient airflow, and EMIshielding, while using up substantially all available “donut shape” fanblade area down to a tiny fraction of a square inch.

Note that the numerical and letter designations assigned to the elementsof FIG. 1 do not connote any type of hierarchy; the designations arearbitrary and have been used for purposes of teaching only. Suchdesignations should not be construed in any way to limit theircapabilities, functionalities, or applications in the potentialenvironments that may benefit from the features of fan tray assembly 10.It should be understood that the fan tray assembly 10 shown in FIG. 1 issimplified for ease of illustration.

Turning to FIGS. 2A and 2B, FIGS. 2A and 2B are simplified diagramsshowing details of an individual perforation 22 according to anembodiment of fan tray assembly 10. Perforation 22 includes twonon-parallel straight lines 24 and 26 and two large curvature lines 28and 30. Lines 24 and 26 may originate at an imaginary circle centered atfan 12's center, to form an angle with its vertex at the center. Curvedlines 28 and 30 may be portions of concentric circles with their commoncenter corresponding to the fan's center. Perforation 22 may bepatterned in small angular increments around fan 12's center.Perforations 16 may be patterned in concentric circles, forming layersfrom a small circle corresponding to an inner radius of fan 12 to alarge circle corresponding to an outer radius of fan 12.

FIG. 2B shows perforation 22 in detail. Perforation 22 includes fouredges 24, 26, 28, and 30. Edges 24 and 26 may be portions ofnon-parallel angular lines with their vertex at fan 12's center. Edges28 and 30 may be portions of concentric circles having their commoncenter at fan 12's center. Perforation 22 may be rounded out at thecorners by curves 32(1)-32(4). In some embodiments, curves 32(1)-32(4)may have any suitable radius that provides a pleasing aestheticappearance.

Turning to FIG. 3, FIG. 3 is a simplified diagram of a fan and pattern20 of perforations 16 according to an embodiment of fan tray assembly10. Fan 12 may comprise blades 40 attached to an approximately circularinner portion 42. Fan 12 may have an approximately circular outercontour with a radius R_(outer). A rotary part of fan 12 (e.g.,including fans 40) may have an inner radius R_(inner) (corresponding tothe radius of the inner portion). Pattern 20 may be bounded on an innerperimeter by an imaginary circle with radius R_(inner), and bounded onan outer perimeter by another imaginary circle with radius R_(outer),both circles being centered on a common point C. Perforations 16 may bearranged in (imaginary) concentric circles forming layers 44(1)-44(n),all centered on point C, with the smallest concentric circle havingradius greater than or equal to R_(inner), and the largest concentriccircle having radius smaller than or equal to R_(outer).

Turning to FIGS. 4A to 4F, FIGS. 4A to 4F are simplified diagramsillustrating an example method to create perforation 22 and pattern 20according to an embodiment of fan tray assembly 10. Initially, cutoutsare drawn according to pattern 20, where each cutout corresponds toperforation 22. As used herein, the term “cutout” can include arendering of a perforation, for example, drawn on paper, displayed on acomputer screen, etc. In FIG. 4A, two circles 46 and 48 may be drawnaround point C, with radii R_(A) and R_(B) (R_(B)>R_(A)), such thatR_(A)<R_(inner) and (R_(B)−R_(A)) corresponds to D, a length ofperforation 22. In an example embodiment, R_(A)=2.260″ and D=0.110″. Athird circle 50 of diameter D may be drawn between circles 46 and 48,centered at a distance (R_(A)+R_(B))/2 from point C.

In FIG. 4B, two non-parallel lines 52 and 54 may be added, forming anangle with vertex at point C. Lines 52 and 54 may be tangent to circle50. FIG. 4C shows circle 50 in detail. Circles 46 and 48 and lines 52and 54 may be tangential to circle 50. In FIG. 4D, excess portions(e.g., that are not near circle 50) of circles 46 and 48 and lines 52and 54 may be erased (e.g., trimmed) to generate a rough outline ofcutout 56. As shown in detail in FIG. 4E, a final outline of cutout 56may be generated by rounding the corners of the rough outline withcurves 32(1)-32(4). In the embodiment illustrated in FIG. 4E, radius ofcurves 32(1)-32(4) is 0.035″. Any suitable radius may be chosen forcurves 32(1)-32(4) within the broad scope of the embodiments.

Turning to FIG. 4F, cutout 56 may be replicated X₁ times (e.g., 50times) around point C along an imaginary circle 60 of radius R₁(=(R_(A)R_(B))/2), with each cutout uniformly angularly spaced by angleA1 from the adjacent cutouts. In some embodiments, X₁ may be calculatedas the nearest integer less than the perimeter (2πR₁) of circle 60. Inother embodiments a distance d between cutouts may be predetermined(e.g., according to thickness of plate 14, and for other manufacturingconsiderations) and X₁ may be calculated as the total integer number ofcutouts spaced d apart that can be placed along circle 60. Angle A₁ maybe calculated as 360°/X₁. Cutouts along circle 60 may correspond to afirst layer 44(1) of perforations 16 in pattern 20.

Turning to FIG. 4G, cutout 56 may be replicated on another circle 62concentric with circle 60, centered at C, with radius R₂=R₁+Δ, where Δis a suitably small number. In some embodiments, Δ may be calculatedbased on the distance d between cutouts and size of cutout 56 (e.g.,Δ=D+d). Cutout 56 on circle 62 may be replicated X₂ number of times,each cutout uniformly angularly spaced at angle A₂ from each other. Insome embodiments, A₂ may be smaller than A₁, so that cutouts are spacedd apart azimuthally and radially. Cutouts along circle 62 may form asecond layer 44(2) of perforations 16 in pattern 20.

Similarly, cutout 56 may be replicated on yet another circle 64 centeredat C with radius R₃=R₂+Δ. The cutouts in circle 64 may be uniformlyangularly spaced at angle A₃ from each other (A₃<A₂<A₁), with X₃ numberof cutouts in circle 62. Cutouts along circle 64 may form a third layer44(3) of perforations 16 in pattern 20. The cutouts may be replicateduntil pattern 20 is filled (e.g., radius of largest concentric circleexceeds R_(outer) of fan).

In various embodiments, pattern 20 may be transferred to a manufacturingequipment (such as a stamp, or laser cutter), and perforations 16generated on plate 14 according to pattern 20. Plate 14 may be composedof any suitable material (e.g., metal, plastic, wood, fibers, etc.). Insome embodiments, where plate 40 and fan 12 are used in switches andother electronic equipment, plate 14 may be made of metal, to enhanceEMI shielding performance.

Turning to FIG. 5, FIG. 5 is a simplified flow diagram illustratingexample operations that may be associated with generating pattern 20according to an embodiment of fan tray assembly 10. Operations 100 maystart at 102 and proceed to 104, at which circular fan 12 may beprovided with inner perimeter of radius R_(inner) and outer perimeter ofR_(outer) from center C. At 106, first circle 46 may be drawn withcenter C and radius R_(A) (R_(A)≧R_(inner)). At 108, second circle 48may be drawn with center C and radius R_(B), such that R_(B)>R_(A) andR_(B)≦R_(outer), and (R_(B)−R_(A)=D). At 110, third circle 50 may bedrawn between circles 46 and 48 with diameter D and center at a distance(R_(A)+R_(B))/2 from C.

At 112, two non-parallel lines 52 and 54 may be added, originating atcenter C and tangent to circle 50, to create a rough outline of cutout56. At 114, excess lines and large circles may be trimmed outside therough outline of cutout 56. At 116, rounded corners may be added torough outline 56 to create a final outline of cutout 56. Cutout 56 maybe centered at distance (R_(A)+R_(B))/2 from center C. At 118, a counteri may be set to 1. At 120, a radius R_(i) may be set to (R_(A)+R_(B))/2,corresponding to the distance of cutout 56 from center C. At 122, adetermination may be made whether R_(i)>R_(outer).

If R_(i) is not greater than R_(outer), at 124, X_(i), the number ofcutouts that can fit in circle 60 with center C and radius R_(i), andspaced d apart may be calculated. At 126, angle A, between adjacentcutouts on circle 60 may be calculated as 360°/X_(i). At 128, cutout 56may be replicated Xi number of times at angle A_(i) around center C. Thecutouts on circle 60 may correspond to layer 40(1) of perforations 16 inpattern 20. At 130, Δ may be calculated as D+d. At 132, counter i may beincremented by 1 to i+1. Radius R_(i) may be calculated to be Ri⁻¹αΔ.The operations may loop back to 122 and continue until R_(i)>R_(outer),at which point, the operations may end at 138, when pattern 20 iscompleted.

Turning to FIG. 6, FIG. 6 is a simplified flow diagram illustratingexample operations that may be associated with an embodiment of fan trayassembly 10. Operations 150 may include 152, at which a drawing ofpattern 20 may be created (e.g., according to operations 100). At 154,the drawing of pattern 20 may be transferred to suitable manufacturingequipment (e.g., stamp, forge, lathe, computer numerical controlled(CNC) machine, laser cutter, driller, etc.). At 156, perforations 16 maybe generated on plate 14 according to pattern 20.

Turning to FIG. 7, FIG. 7 is a simplified flow diagram illustratingexample operations that may be associated with an embodiment of fan trayassembly 10. Operations 160 may include 162, at which fan 12 may berotated. At 164, air may be guided through plurality of perforations 16in plate 14 located behind fan 12, such that air flows from fan 12towards plate 14.

In terms of the dimensions of the articles discussed herein (e.g., thefan, the plate, the pattern, etc.), any suitable length, width, anddepth (or height) may be used and can be based on particular end userneeds, or specific elements to be addressed by the apparatus (or thesystem in which it resides). It is imperative to note that all of thespecifications and relationships outlined herein (e.g., height, width,length, hole diameter, # holes per unit of area, etc.) have only beenoffered for purposes of example and teaching only. Each of these datamay be varied considerably without departing from the spirit of thepresent disclosure, or the scope of the appended claims. Thespecifications apply only to one non-limiting example and, accordingly,should be construed as such. Along similar lines, the materials used inconstructing the articles can be varied considerably, while remainingwithin the scope of the present disclosure.

Note that in this Specification, references to various features (e.g.,elements, structures, modules, components, steps, operations,characteristics, etc.) included in “one embodiment”, “exampleembodiment”, “an embodiment”, “another embodiment”, “some embodiments”,“various embodiments”, “other embodiments”, “alternative embodiment”,and the like are intended to mean that any such features are included inone or more embodiments of the present disclosure, but may or may notnecessarily be combined in the same embodiments.

It is imperative to note that countless possible design configurationscan be used to achieve the operational objectives outlined here.Accordingly, the associated infrastructure of fan tray assembly 10 mayhave a myriad of substitute arrangements, design choices, devicepossibilities, hardware configurations, equipment options, etc. It isalso important to note that the operations and steps described withreference to the preceding FIGURES illustrate only some of the possiblescenarios that may be executed by, or within, the system. Some of theseoperations may be deleted or removed where appropriate, or these stepsmay be modified or changed considerably without departing from the scopeof the discussed concepts. In addition, the timing of these operationsmay be altered considerably and still achieve the results taught in thisdisclosure. The preceding operational flows have been offered forpurposes of example and discussion. Substantial flexibility is providedby the system in that any suitable arrangements, chronologies,configurations, and timing mechanisms may be provided without departingfrom the teachings of the discussed concepts.

Although the present disclosure has been described in detail withreference to particular arrangements and configurations, these exampleconfigurations and arrangements may be changed significantly withoutdeparting from the scope of the present disclosure. For example,although the present disclosure has been described with reference to afan tray, fan tray assembly 10 may be applicable to other devices wherea similar pattern of holes may be desired.

Numerous other changes, substitutions, variations, alterations, andmodifications may be ascertained to one skilled in the art and it isintended that the present disclosure encompass all such changes,substitutions, variations, alterations, and modifications as fallingwithin the scope of the appended claims. In order to assist the UnitedStates Patent and Trademark Office (USPTO) and, additionally, anyreaders of any patent issued on this application in interpreting theclaims appended hereto, Applicant wishes to note that the Applicant: (a)does not intend any of the appended claims to invoke paragraph six (6)of 35 U.S.C. section 112 as it exists on the date of the filing hereofunless the words “means for” or “step for” are specifically used in theparticular claims; and (b) does not intend, by any statement in thespecification, to limit this disclosure in any way that is not otherwisereflected in the appended claims.

1.-20. (canceled)
 21. A method, comprising: guiding air through aplurality of perforations on a plate proximate to a rotating fan,wherein the perforations are arranged in concentric circles centered ata point, wherein each of the perforations is a closed shape comprisingfour edges that are tangential to a hypothetical circle located insidethe perforation, wherein two opposite edges of each perforation comprisenon-parallel straight lines, and two other opposite edges comprisecurved lines.
 22. The method of claim 21, wherein the fan issubstantially circular with an inner radius corresponding to an innerperimeter of the pattern and an outer radius corresponding to an outerperimeter of the pattern.
 23. The method of claim 21, wherein eachperforation comprises at least a first edge, a second edge, a third edgeand a fourth edge, wherein the first edge and the second edge comprisethe non-parallel straight lines, wherein the third edge and the fourthedge comprise concentric circles of a first radius and a second radiusrespectively.
 24. The method of claim 21, wherein each perforation hasrounded edges.
 25. The method of claim 21, wherein each perforation isseparated by uniform angle increments in an azimuthal direction fromadjacent perforations located on a common concentric circle.
 26. Themethod of claim 21, wherein the perforations are spaced apart equally inboth azimuthal and radial directions.
 27. An apparatus, comprising: aplate comprising a plurality of perforations arranged in concentriccircles centered at a point, wherein each of the perforations is aclosed shape comprising four edges that are tangential to a hypotheticalcircle located inside the perforation, wherein two opposite edges ofeach perforation comprise non-parallel straight lines, and two otheropposite edges comprise curved lines.
 28. The apparatus of claim 27,wherein each perforation comprises at least a first edge, a second edge,a third edge and a fourth edge, wherein the first edge and the secondedge comprise the non-parallel straight lines, wherein the third edgeand the fourth edge comprise concentric circles of a first radius and asecond radius respectively.
 29. The apparatus of claim 28, wherein adiameter of the hypothetical circle is equal to a difference between thefirst radius and the second radius.
 30. The apparatus of claim 27,wherein each perforation is separated by uniform angle increments in anazimuthal direction from adjacent perforations located on a commonconcentric circle.
 31. The apparatus of claim 27, wherein theperforations are spaced apart equally in both azimuthal and radialdirections.
 32. The apparatus of claim 27, wherein any two perforationsare uniformly spaced from each other.
 33. The apparatus of claim 27,wherein the pattern is bounded within an inner circle having an innerradius and an outer circle having an outer radius, wherein the apparatusfurther comprises a circular fan having the inner radius and the outerradius, wherein the plate is located parallel to the fan.
 34. Theapparatus of claim 33, wherein rotation of the fan causes air to bepushed out through the plurality of perforations.
 35. The apparatus ofclaim 27, wherein the plate is made of a material providingelectromagnetic interference (EMI) shielding.
 36. The apparatus of claim27, wherein the apparatus is attached to electronic equipment.
 37. Theapparatus of claim 36, wherein the electronic equipment comprises aswitch in a network.
 38. A method, comprising: generating a pattern on aplanar surface, the pattern comprising a plurality of perforationsarranged in concentric circles centered at a point, wherein each of theperforations is a closed shape comprising four edges that are tangentialto a hypothetical circle located inside the perforation, wherein twoopposite edges of each perforation comprise non-parallel straight lines,and two other opposite edges comprise curved lines; and cutting out theplurality of perforations.
 39. The method of claim 38, wherein theconcentric circles have mutually different number of perforations. 40.The method of claim 38, wherein each perforation has rounded corners.